Earlier (with Outlook 2010 32bit and Gpg4win 3.1.x < 3.1.2) using the option "Neue Nachrichten per Voreinstellung signieren" (Sign new mails by preference) would sign mails when answering / forwarding.
Now (with Outlook 2013 64bit and Gpg4win 3.1.2) signing is preselected only with new mails, but not when answering / forwarding.